In addition to being a developer/entrepreneur, I moonlight as a high school marching band instructor/arranger. In other words, I'm a semi-professional composer. I work with a team of people who write different aspects of the music: One guy writes the winds, I write the percussion, and others contribute as well.
Currently, whenever one of us has an idea about something to do with whatever we're arranging, we have to either get everyone together in a room with a piano to talk through it, or someone has to sit down with a professional music notation program (we use Finale) and "sketch" it all out.
It would be really cool if there was something online like RiffUp that would let me put together that "sketch" quickly and easily, and send a link to my colleagues. Like a "pastebin" for sheet music.
To get there, it would need to do a few more things, in roughly priority order: * Allow you to save your creation and send a link to someone else. * Support "-let" rhythms (triplets, for example) * Use multiple staves * Set the tempo * Allow other time signatures (time changes would be good too) * Choose a different voice for each staff
The great benefit here would be simple composition without having to worry about all of the stuff associate with making a real score. So much of music notation is about making stuff that prints nicely, which is actually a separate process from just creating the music. And, of course, being able to easily share it with colleagues without requiring them to have the same software I do (or to download a file and open it in that software).
And to answer the followup question that you'll ask: Yes, I'd be willing to pay for this.